A rear projection display, in which a projector projects image light obliquely on a rear projection screen, is provided with a Fresnel lens sheet proposed in JP 61-208041 A (Patent document 1). This Fresnel lens sheet is an optical element for converging the light obliquely fallen on the rear projection screen. The Fresnel lens sheet has an entrance surface provided with prisms having a triangular cross section. Incident light fallen on the entrance surface is refracted at a first facet of each prism, and the refracted light is totally reflected by a second facet of the prism and travels outside through an exit surface opposite the entrance surface. This Fresnel lens sheet is capable of efficiently making the light obliquely fallen on the entrance surface go out through the exit surface. Therefore, the Fresnel lens sheet incorporated into the rear projection display is effective in reducing the size (the depth) of the rear projection display and in forming the rear projection display in a small size.
However, this Fresnel lens sheet has a problem that contrast in images is liable to be reduced by external light fallen on the exit surface of the Fresnel lens sheet in a bright room or such. Such a problem arises in a Fresnel lens sheet 30 having a base part 31 having an entrance surface provided with prisms 32 each having a refraction facet 32a and a total-reflection facet 32b, and an exit surface 31b as shown in FIG. 9 because some external light rays L2 among those fallen on the exit surface 31b are refracted several times by the refraction facets 32b and total-reflection facets 32b of the prisms 32 and travel outside through the exit surface 31b of the base part 31.
In this Fresnel lens sheet 30, some light rays L3 among those fallen at small incident angles on the entrance surface of the base part 31 are not reflected totally by the total-reflection facets 32b of the prisms 32, and the light rays L3 are reflected by the exit surface 31b of the base part 31 and travel outside from different positions on the exit surface 31b as shown in FIG. 10. Consequently, a double image is formed.
A known method of solving those problems proposed in JP 63-30835 A (Patent document 2) forms V grooves in regions, through which light rays L1 fallen on the entrance surface of a base part 31 do not pass, of the exit surface 31b of the base part 31 of a Fresnel lens sheet 30, and forms V-shaped light absorbing parts 34 on the slopes of the V grooves 33, respectively, as shown in FIG. 11.
The method mentioned in Patent document 2, however, needs to form the light absorbing parts in regions, through which light rays fallen on the entrance surface of the base part do not pass, of the exit surface of the base part. Therefore, the positional relation between the prisms on the entrance surface of the base part and the light absorbing parts on the exit surface must be properly adjusted. Generally, the pitches of the prisms on the entrance surface are on the order of 0.1 mm. Therefore, the positions of the prisms and those of the light absorbing parts must be determined relative to each other in a positioning accuracy of about 0.01 mm or higher. It is very difficult to manufacture such a Fresnel lens sheet. If the prisms extend parallel to a straight line, the positions of the light absorbing parts parallel to the straight line with respect to a direction perpendicular to the straight line are adjusted. However, if the prisms extend along circular arcs, the positions of the light absorbing parts need to be adjusted with respect to two perpendicular directions. Such positioning is very difficult.
If the positions of the prisms on the entrance surface of the base part and those of the light absorbing parts on the exit surface of the base part are adjusted relative to each other in a low positioning accuracy, moiré occurs due to positioning errors in both a case where the prisms extend parallel to a straight line and a case where the prisms extend along circular arcs.